Slain US hostage was repeatedly raped by ISIL leader

An American aid worker who was murdered by ISIL in Syria earlier this year was repeatedly raped by the group's leader before her death, it has emerged.

Kayla Mueller, 26, was killed in February after being taken captive by the extremist force in August 2013 while working in an Aleppo hospital run by Médecins Sans Frontières.

Her family learned in June of the abuse she suffered at the hands of ISIL leader Abu Bakr al-Baghdadi, who is known to keep a harem of sorts at his house in al-Shadidiya, after a 14-year-old Yazidi sex-slave who met Ms Mueller in captivity escaped and told the story, which has been confirmed by US officials.

The new information is a chilling reminder of ISIL's use of rape as both a tool of intimidation as well as a "reward" for victorious soldiers; scores of Yazidi women (ethnic Kurds who subscribe to an ancient religion) have been abducted and traded as sex-slaves.

Abu Bakr al-Baghdadi. (AP) ()

According to the 14-year-old escapee, Ms Mueller was raped – or "married by force" – by several ISIL leaders, including Abu Bakr.

"I saw him often, he came there often," the Independent reports the Yazidi witness as saying.

"And every time he came, he took Kayla away. She would always come back after."

The teen says Ms Mueller would tell her and the other three girls in the room, often through tears, that she had been "married by force".

The girl also said that Ms Mueller was a courageous "mother figure" to the wretched group, and often put herself at risk to protect them from Abu Sayyaf, who was an ISIL financier, and his wife, Umm Sayyaf, who has hands-on involvement in the trafficking of Yazidi sex-slaves.

Abu Sayyaf was killed in the raid on the house carried out by American forces in May which led to the emancipation of the 14-year-old Yazidi who remembers Ms Mueller so fondly, and his wife was captured in the attack.

When he announced her death in February, US President Barack Obama said that Ms Mueller had "epitomised all that is good in our world."